text:
What happens in a stall during a slip?
In flight training we're warned against skidding turns since they have a higher potential for a stall/spin (the classic example being the stall/spin on the base to final turn).
However, how does the airplane behave during a stall entered from a slip?
It's a cross-controlled condition, but since the rudder input is opposite to what it would be in a skid, is it more difficult, or not possible, to spin from such a situation?

A stall could turn into a spin any time you are uncoordinated.

@FredLarson That isn't exactly true....

In order for a spin to develop, you need two things from the airplane:

it must be stalled
it must be yawing.

Since the airplane is not yawing while in a slip (it is actually flying straight ahead, although uncoordinated), the airplane simply won't spin.  Instead, when the stall occurs the higher wing (which has the higher angle of attack) will stall first, the wing will drop, and the wings will tend level.
That being said, it is still a stall and can be dangerous at low altitudes.  It can also turn into a spin if you leave the rudder applied as the wings level and you come out of the slip (and into a skid).
The best protection for all of this is to watch your airspeed and angle of attack to make sure that you always have sufficient airspeed so that you never stall in the first place.
Additional Information
Since this has generated some debate, here is additional information:
Spin
According to Wikipedia (emphasis added by me):

Spins can be entered intentionally or unintentionally, from any flight
  attitude and from practically any airspeed—all that is required is
  sufficient yaw rate while an aircraft is stalled.

Yaw
So this brings us to Yaw - Wikipedia:

The yaw axis is defined to be perpendicular to the body of the wings
  with its origin at the center of gravity and directed towards the
  bottom of the aircraft. A yaw motion is a movement of the nose of the
  aircraft from side to side.

Skid
Skid - Wikipedia actually covers this exact scenario:

The skid is more dangerous than the slip if the airplane is close to a
  stall. In the slip, the raised wing — the left one if the airplane is
  turning to the right — will stall before the lowered one, and the
  airplane will reduce the bank angle, which prevents the stall. In the
  skid, the lowered wing will stall before the raised one, and the
  airplane will tighten the turn, and the stall can develop to a spin.

Slip
Slip - Wikipedia says:

A slip is an aerodynamic state where an aircraft is moving somewhat
  sideways as well as forward relative to the oncoming airflow or
  relative wind. In other words, for a conventional aircraft, the nose
  will be pointing in the opposite direction to the bank of the wing(s).
  The aircraft is not in coordinated flight and therefore is flying
  inefficiently.

Note that the aircraft is moving sideways.  The nose of the aircraft is not moving, and it is therefore not yawing.  It also says:

Often, if an airplane in a slip is made to stall, it displays very
  little of the yawing tendency that causes a skidding stall to develop
  into a spin. A stalling airplane in a slip may do little more than
  tend to roll into a wings level attitude. In fact, in some airplanes
  stall characteristics may even be improved.

actually, you can develop a spin by using a single item: one wing must be stalled. The ensuing drag imbalance will induce the yaw required and, if not checked, the airplane will/might evolve into a full spin. Slipping turns and slips are both good occasions to stall a single wing, although I agree that getting into a spin from a slip could take quite some work

@Radu094 Well, as I said in my answer, if you leave the rudder in after the stall, the slip will turn into a skid and you can then spin the airplane.

In order for an airplane to spin, it must be stalled and in uncoordinated flight. A slip is uncoordinated flight.  I think you are confusing ground track and track through the air.  Typically a pilot will perform a "forward slip" to counter a crosswind when landing but this straight flight (ground track) is uncoordinated flight - just look at the "ball".  So spins from slips are very possible. They are much less likely than spins from skids.  The reason is that in a skid, the bottom wing stalls first dropping the plane into a spin, whereas in a skid the top wing stalls first and drops.

(continued) which actually gives the pilot considerably more time to correct and recover.

@SkipMiller I am not contesting the fact that a slip is uncoordinated (I even say that it is in my answer).  However, you are not yawing when in a slip.  Yaw: A yaw motion is a movement of the nose of the aircraft from side to side."  In a slip, the nose of the aircraft is not moving at all: It is steady and is not yawing.  In fact, you are affecting the ground track (which is why you do it to counteract the wind).  If you feel that you can provide a better answer, feel free to add your own though, that's the premise of our whole system here!  :-)

@SkipMiller I have updated the answer with additional information.

Louis, my problem with your answer is that the nose of your aircraft is yawing in a coordinated turn.  But as long as you remain coordinated, you will not spin.

@SkipMiller In a coordinated turn, the entire airplane is turning at a rate appropriate for the angle of bank. In a yaw, the nose of the airplane is pivoting around the vertical axis.  It sounds like you are misunderstanding what exactly a yaw is.  I would suggest doing a little bit of reading specifically about yaw!  :)

The definition posted above (from that authoritative source Wikipedia!) does not make that distinction.

@SkipMiller Well, that's only part of the explanation.  I'll try to update it with more details later (I was actually thinking about that while driving this morning and want to pull up the fundamentals of flight or something and quote a better source.).  :)

I think the following video is relevant here to show the possibility of a spin from a slip with very little or no yaw: https://www.youtube.com/watch?v=rYHNO2kvuZI&t=185 (from 2:55)

@JulianHzg I just noticed your comment, lol.  That video description specifically says that the spins are entered due to a yawing turn with too much rudder.  That is the exact point that I am trying to make:  You must be yawing in order to spin.

You are indeed yawing in a coordinated turn, yes or no?
If you stall in a coordinated turn, will you enter a spin?  I'll answer that one: No.  My CFI and I spent a few hours exploring accelerated stalls, both straight and turning.  This is not a spin entry maneuver.
So the original statement you made that two things are necessary for a spin: stalled and yawing, is not completely correct.  It is stalled and uncoordinated.  I don't think this is just a nit we are picking.  Having said that, spins out of slips are rare.  In an aggressive slip, the nose can blank the airflow over one wing and if the pilot doesn't react immediately and appropriately, you are in a spin.  I discount the logic that a badly executed slip turns into a skid turns into a stall.  While possibly true aerodynamically, the purpose of our training is to prevent unintentional spins, and you can indeed get into a spin from a slip.  Your statement that you can't spin from a slip is just not helpful to the majority of pilots who are basically just in search of the Hundred Dollar Hamburger.

Sorry Skip, but you do need both of those to enter a spin, and in a coordinated turn you are indeed correct that it won't spin, but that is because it isn't yawing.

I've been quiet for a while because I recognize the possiblity that I have a less than complete understanding of this.  But this has been gnawing at me so please educate me.  You are engaged in a full rudder slip.  Now you relax a bit the rudder pressure, and you are still in a slip but now the nose is moving across the horizon.  Is that a yaw?  Now you stall.  If what you say is true, the plane will not enter a spin because you are in a slip.  Please explain why not?  And thanks for your patience.

So what you describe there could indeed turn into a spin because if the nose starts to move across the horizon, you have started to yaw (aka skid).  You would have transitioned from a slip to a skid because you only "relaxed a bit of rudder pressure" instead of maintaining the zero yaw condition (slip) that you were in before.  In my answer above I say "It can also turn into a spin if you leave the rudder applied as the wings level and you come out of the slip (and into a skid)."

I am a Full Time CFII for 10 years now with 8000 flight hours. You are infact correct. But you already know that. I don't understand why lnafziger does not think you are correct. he was saying that if the airplane is stalled and yawing you will spin. Obviously thats incorrect. He doesn't recognize that you are infact yawing while turning. But that doesn't mean you are uncoordinated while doing so. The simple answer as you stated earlier and i'll say it again is it takes two things to spin: A stall and Un-Coordinated flight. Simple as that.

A skidding turn is more prone to spins than a slipping turn, but you can enter a spin from a slip as well.
In a slip, the high wing stalls first. This tends to roll the plane towards wings level. But there's nothing that says the roll has to stop at wings level- it's possible for the roll to carry straight through wings level and into an incipient spin.


text:
Can someone explain Exemption 3585?
Exemption 3585 allows 121 carriers (and perhaps others) to dispatch to destinations in certain cases with weather below minimums.
What are the forecast weather minimums to be dispatched under exemption 3585?  When is it required to have a second alternate listed on the release?  What are the forecast weather requirements at the two alternates?  What is the regulatory basis for this exemption?

Exemption 3585 is an exemption to 14 CFR 121 dispatch requirements for weather originally requested by Peoples Express.  Lots of airlines have requested similar approval, and here is an example on regulations.gov for Air Transportation International.
The exemption that they asked and were approved for was:

...an exemption from §§ 121.613, 121.619(a), and 121.625 of Title
  14, Code of Federal Regulations (14 CFR) to the extent necessary to
  dispatch airplanes under instrument flight rules when conditional
  language in a one-time increment of the weather forecast states
  that the weather at the destination, alternate airport, or both
  airports could be below the authorized weather minimums when other
  time increments of the weather forecast state the weather conditions
  will be at or above the authorized weather minimums.

In other words, if there is a conditional or temporary weather condition that is below minimums at the destination or the alternate airport at the flight's ETA1,2, where the flight could not normally be released, they can depart under the 3585 exemption as long as a second alternate that is above required minimums3 is filed in the flight plan and enough fuel is carried to reach the furthest alternate plus the required 45 minute reserve.  There are a few other requirements too (training, manual and OpSpec revisions, etc.) but you can take a look at the letter to see them.
The exemption must be requested of and approved by the FAA for each air carrier that wants to use it.

1 The forecast weather conditions at the destination airport must not be less
than one-half of the lowest weather minimum visibility value established
for the instrument approach procedure expected to be used at the
destination
2 The forecast weather conditions at the first alternate airport must not be less than one-half of the alternate weather minimum ceiling and visibility values
specified in ATI’s operations specifications for that airport
3 With respect to the second alternate airport listed in the flight release
pursuant to this exemption, the METAR, the appropriate time increment of
the TAF, or any combination thereof for that airport must indicate in the
main body and remarks section of the METAR or TAF that the forecast
weather conditions will be at or above the alternate airport weather
minimum ceiling and visibility values specified in ATI’s operations
specifications for that airport

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text:
What happens when I tell ATC to "mark the tape"?
You can find anecdotal mention of this phrase among pilots you may talk to as well as postings on various forums.  The idea conveyed is that telling a controller to "mark the tape" means they will make take some action to note the current time for the audio recording of his seat, which can then be referred back to by someone.  As many anecdotes you can find about this, you'll find just as many saying this phrase is essentially an old wives tale, and means absolutely nothing.
Given a situation with a controller that I'd like to bring to the attention of his superiors, or perhaps just with him over the phone, does using the phrase "mark the tape" do anything?  If it does nothing, what is the proper procedure I should use to review something with a controller or their supervisor (e.g. noting time and frequency, asking for a phone number)?

I don't know the answer, but if it does mean something I do know it's not necessary. If you know the time of the event that's all a manager needs to pull the tape. I have done this (when I wanted a clarification of a minor point that came up while I was flying). It took the manager about half an hour to find and listen to the tape and call me back.

@dvnrrs I feel the same way, this Q is partially asked to debunk that phrase and the need to use it.

These days, with digital recording, it's likely that nothing special happens.
"Mark the tape" is a holdover from when ATC recordings were actually on tapes - you could ask a controller to "mark" a tape to hold it and prevent it being recycled for another recording while your request for a copy of the tape is being processed. (Anecdotally I've been told some facilities had tape rotations as short as 15 days, and a FOIA request letter could easily take longer than that to make it to the facility).
Today much (probably most) of what is said on the frequency is recorded digitally, direct to hard drives, and kept around "essentially forever" as one of our local controllers put it when this came up at a recent event.  Basically the virtual "tapes" stick around for as long as the facility's recording machine has disk space to store them, and since audio files aren't that big facilities can store recordings a lot longer than they did in the days of tape.

All that's really needed to pull a record to review it with a controller/supervisor is the date, time, facility, and frequency (and your aircraft callsign would probably be helpful too) - e.g. "April 4, 2014 - JFK Local Control for Runway 04R (119.1) - United 123 Heavy".
"Mark the tape and give me a phone number" survives as non-standard phraseology that lets the controller know that you will likely have a bone to pick with someone when you get on the ground and gives them the opportunity to tell their supervisor that an annoyed pilot will be calling the facility in the near future (and hopefully give the supervisor time to listen to the tape).
The useful part is really getting the facility phone number so you know who to call when you land.

As recently as 2009, when I last worked at ZME, they actually still recorded audio on high density magnetic tape.  These tapes were kept in a secure vault that requires two persons to open, and this is only done under the rarest of occasions.  The physical tape is not marked, as you said, but the tape is held.

"Mark the tape" is a hold-over term from way back when the recorders were reel-to-reel tapes. In order to mark the tapes, the controller would place a flight strip in the reel at the time of the occurence. As previously stated, with almost everything being digital, it's unnecessary.

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text:
Can I get official recordings of ATC audio?
Is there a procedure in place to request a copy of the audio recording of a specific ATC position over a specific time interval?  How is the request made? What are the limitations of such a request?
For the purpose of this answer, consider that there would be a need for the audio from a controller, which may involve multiple frequencies as well as interphone communication with other controllers.  As such, internet resources that provide recordings of specific frequencies would not be adequate.

Note for completeness: an FAA recording will capture everything that passed though the position, which means: Any RX transmission on any selected frequency, any TX transmission no matter which frequencies are selected (if any are), any incoming audio as well as any audio (intentional or background) captured by the controller's microphone while a landline was active, and any audio captured by either controller's microphone while the "brief button" was active. To my knowledge this will all be mixed down into a single audio channel with no metadata identifying the original source.

In the specific situation of a pilot who is the subject of an FAA investigation, the Pilot's Bill of Rights requires that air traffic data (which includes audio recordings) be provided to the pilot. They would be expected to make the request directly to the FAA. There is a procedure for this:

To ensure that relevant air traffic data you are seeking from a government contractor are obtained, you should provide the following information:
a. Your name and your airman certificate number;
b. other relevant information (aircraft heading, altitude, call sign, transponder code, etc.) that will help the contractor locate relevant data;
c. the name/location of the contractor facility that you believe has air traffic data;
d. the date and local time of day when such data relevant to the your flight operation were generated;
e. the name of the FAA inspector, investigator or other FAA official (e.g., an air traffic controller who advises you about a possible pilot deviation) who notified you that you are under investigation;
f. the airman's phone number or email
The individual should provide that information to the FAA by email to the following address: AirmenDataRequest@faa.gov.

The instructions include a warning about timeliness:

Depending on the type of data, they will normally be stored from 5 to 45 days and then they are destroyed or discarded in the normal course of business

I don't have any information about the general case, though I imagine it depends on the facility and their workload.

In "the general case" you make your request under the Freedom of Information Act (FOIA). Information on how these requests are handled can be found in FAA Order JO 7210.3X (Section 4.8)

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text:
When do I need a takeoff alternate?
Under what conditions do I need to have a takeoff alternate airport listed on a dispatch release?  How far away can my takeoff alternate airport be?  Do I need to worry about this in part 91 operations or is it just a 121/135 thing?

You should refer to the published IAPs for an airport to determine if it has non-standard takeoff minimums (for AeroNav charts, this is the T in the black triangle). There's a section on this in the Instrument Procedures Handbook, Chapter 2 (2-6):

The FAA establishes takeoff minimums for every airport that has published Standard Instrument Approaches. These minimums are used by commercially operated aircraft, namely Part 121 and 135 operators.

14 CFR 135.217 describes the requirements for the alternate airport:

No person may takeoff an aircraft under IFR from an airport where weather conditions are at or above takeoff minimums but are below authorized IFR landing minimums unless there is an alternate airport within 1 hour's flying time (at normal cruising speed, in still air) of the airport of departure.

...and so does 14 CFR 121.617:

(1) Aircraft having two engines. Not more than one hour from the departure airport at normal cruising speed in still air with one engine inoperative.
(2) Aircraft having three or more engines. Not more than two hours from the departure airport at normal cruising speed in still air with one engine inoperative.

Part 91
14 CFR 91 does not require a takeoff alternate. I'm going to quote the IFH again because it's an authoritative FAA source and because I very much agree with it (2-6):

Aircraft operating under Part 91 are not required to comply with established takeoff minimums. Legally, a zero/zero departure may be made, but it is never advisable.


text:
How are turbine engines started?
What happens to a turbine engine from the time that a start is initiated until stable idle is achieved?
What can go wrong?

Of course, everyone knows that to start the engine of the Phoenix, you shoot it...

"Of course, everyone knows that to start the engine of the Phoenix, you shoot it": https://www.youtube.com/watch?v=AwYzAF9bZLI

The means of starting most turbine engines is high pressure bleed air.  This bleed source typically comes from an APU but may also come from an already running engine (crossbleed start) or an external huffer cart.
In some airplanes the start is completely FADEC controlled and need only be initiated and monitored by the flight crew, and in other airplanes certain steps must be manually initiated.
The process below best describes a turbofan engine.  Read @Falk s answer for information on differences with turboprop engines.
Turbofan basics
Refer to the image below (Credit: K. Aainsqatsi, Wikipedia):
The portions of the engine I will be referring to below are:

The N1 fan, N1 shaft and N1 turbine which are displayed in green and labelled "Fan/Low-pressure compressor", "Low pressure shaft" and "Low pressure turbine".  These units are connected and move as one piece.
The N2 compressor, N2 shaft and N2 turbine, displayed in purple and labelled "High pressure compressor", "High Pressure shaft" and "High pressure turbine".  These are connected and move as one, but independently of the N1 shaft.  Un-pictured is an accessory drive that is geared to the N2 shaft to drive engine accessories.
The hot section, pictured yellow and labelled "Combustion chamber".  This is home to a constantly burning fire in a jet engine.

Engine Start
The process of the engine starting follows this basic formula

Through the opening of bleed air valves, bleed air is sent to an air turbine starter.  These devices typically use the high pressure bleed air to spin and engage a centrifugal clutch connected to the engines accessory drive.  This in turn causes the N2 shaft within the engine to spin.
With the N2 shaft spinning, the N2 compressor and the N2 turbines are spinning.  This begins to force air through the engine from front to back.
With the accessory shaft and N2 shafts spinning, accessories should start working and this can be verified by oil pressure indications on the EICAS.
With increased N2 rotation, ignition will be turned on.  These igniters are located in the hot section of the engine and produce small sparks.  There should be an indication on the EICAS that the ignition is active.
With further increase in N2 rotation, fuel flow will be introduced.  This will be verified on the EICAS.  Once fuel flow is noted, it is important that the next stop happen fairly soon.
Light off!  The fuel is lit by the ignition and now the fire burning in the hot section supplied by air from the compressor is producing thrust across the N2 and N1 turbines.
As the engine is producing thrust across on the N1 turbine, the N1 shaft is spinning the N1 fan and the EICAS will note this increase in N1 rotation.  N1 and N2 rotation speeds increase.
Above a N2 threshold, bleed air valves supplying the the air turbine starter will close and the starter disengage. The igniters will turn off at some N2 threshold.
The engine will settle into a stable idle thrust setting.

What can go wrong?

Hung start

N2 fails to spin up sufficiently for fuel flow to be introduced.
Each start has a time limit and the start will be aborted.

No ignitors

Abort the start and switch igniters.
Give MX a call.

Failure to ignite

Fuel flow is introduced but light off has not occurred in a proper timeframe
Fuel is building up in the engine and if light off does occur, it can be damaging.
The start is aborted and dry motoring is performed to clear the engine.

ITT exceedence

There is usually a limitation in the inter-turbine temperature during engine start.
If this is exceeded the start must be aborted
This may require coordination with MX before another start is attempted.

The air turbine starter fails to disengage

This must be corrected before departing
In some engines, MX can correct this with the engine running.

The bleed air shutoff valve supplying the air turbine starter fails to close

This may require an engine shutdown to remedy
However, in some airplanes, this valve can be manually shut by MX with the engine running
Coordination with MX required.

Runaway N1 or N2

If the FADEC isn't managing fuel flow properly, the engine may not settle but continue to spin up.
Abort the start.
Coordinate with MX before attempting a restart.

What about starting the engine in the air?
If the engine suffers a flameout, an airborne restart may be attempted.  These starts typically happen one of a few ways:

Crossbleed start
APU start
Windmilling start

The APU start is essentially the same process as above.  The crossbleed start, which can also be done on the ground, merely substitutes a running engine at a high power setting to provide the bleed air for starting and is otherwise the same as above.
The interesting start is the windmilling start.  The necessity for this means something bad has happened.  To need a windmilling start, this means that there are no bleed air sources to supply the air turbine starter.  This can mean that all engines are out and the APU is unavailable (BAD!), or merely that bleed valves to a shutdown engine have failed closed and are unable to be opened.
For the EMB-145 that I am familiar with, a windmilling start required descending at an airspeed between 260 KIAS and 320 KIAS and could not be attempted above FL250.  In short, you hope that the mass flow through the engine is enough to spin the N2 compressor as the ATS would.  With an N2 indication within the engines airstart envelope, you introduce spark and fuel and hope that the engine lights.  In the worst case, if you too slow and unable to provide enough airflow prior to light off, the engine can quickly overtemp and be damaged.  For this reason it is especially important to abort this kind of start as soon as an abnormality is detected.

Just a little gee whiz info, the RB211-22B and -524 as found on the L-1011 had an ENRICH position on the start switch to add a little fuel to get it going and prevent a hung start on cold days. This caused great and exciting white plumes of smoke. See this picture http://www.airliners.net/photo/Trans-World-Airlines/Lockheed-L-1011-385-1-15-TriStar/0061484/L/

What is MX? Google says nothing...

@PavelPetrman maintenance

If you would like to know about jet engines, please read @casey's answer. It's very detailed and good.
There are some turbine engines which are started a different way. We are talking about turboprop or turboshaft engines. A famous example is the PT6 turbine, delivering power to many small turbine-powered propeller airplanes like Beachcraft's King Air series and Piper's Cheyenne and JetProp.
These engines consist of two parts, the gas generator and the free power turbine. The gas generator is pretty much a jet engine. It consists of an inlet, a compressor, an combustion chamber and a turbine. I don't know any turbo prop engine with a nozzle to create thrust, like a jet engine has one, but if one of you knows about it, I would be keen to read your comment. On a turboshaft engine the exhaust isn't at all used to generate thrust so some of them are even inverted (compressor at the rear part of the engine, but of cause still fed with air from an inlet directed forward). You maybe noticed that in some turboshaft driven aircraft the exhaust is very close to the prop - here's the reason.
The second component is a free power turbine, driven by the exhaust of the gas generator, driving the propeller via a reduction gear box (free power turbine speed > 30.000 RPM, prop speed < 3.000 RPM). In most engines these components are only held together by the engine case, that's why it's called free power turbine.
Now we all have a basic idea of this kind of engine and can talk about the start sequence. A so-called starter generator is connected to the gas generator. Basically it's an electric motor powered by a battery (or any other DC power source) used to crank the gas generator shaft. Once the engine is started this motor can be used as a generator delivering DC power. If you look at the principles of an electric motor and a DC generator you'll see that they are technically the same. You only need to change some connections. This is usually done by operating a three position rocker switch 'start-off-generator'.
The start sequence is pretty much the same as it is for a jet engine:

Crank the gas generator's shaft with the starter
At a specific rotation speed (usually 10-15%) add fuel
Monitor the engine parameters
Switch to the generator

Usually the free power turbine starts turning the propeller very soon after fuel is added, anyway some engines are equipped with a propeller brake preventing the free power turbine section from spinning. This is useful to operate the gas generator section only to obtain electric or hydraulic, sometimes pneumatic power - some kind of APU. Aerospaitiale's ATR series is a famous example. The called it "hotel mode".

Thanks for the compliment and broadening the answers to cover more than the turbofan in my answer.

A side effect of starting a free-turbine like the PT-6 is that there is very little torque on the prop during start and idle. King Air pilots are occasionally pranked by passengers going under the wing and holding the prop during start. The pilot sees a running engine but zero prop RPM. You can hang on until about 20% power. Rather less fun with Twin Otters - obvious what's happening.

@paul pics (or video) or it didn't happen ;-)

@paul I suppose http://www.pprune.org/archive/index.php/t-431538.html and others corroborate this but I still would love to see a video ;-)

Many, if not most turboprops have nozzles to create some jet thrust. This is due to the prop efficiency, which is not 100%. As the first approximation, the best (design) ratio of prop vs jet thrust is the same as the prop efficiency in cruise, typically about 85/15. For helicopters it's another story, so in turboshafts they typically use up all the pressure to spin the shaft, sometimes even overexpanding the gas. This requires bigger expanding nozzle, which you can see on many helicopters.

Excellent answers so far.  There are a couple other ways in which a jet engine can be started.
The F-16 has the engine connected to a gearbox with a special, high-speed-balanced shaft.  There is a clutch where the shaft connects to the gearbox.  Also attached to the gearbox is a smaller turbine known as a Jet Fuel Starter (JFS), approximately the size of a large watermelon, also with a clutch. The gearbox also has hydraulic pumps and generators.
To start the plane:

the main engine is declutched from the gearbox
the JFS is clutched to the gearbox
hydraulic pressure is fed from an accumulator to a pump, causing it to work as a hydraulic motor, which turns the gearbox
the turning gearbox spins up the JFS to the point that it reaches starting speed
fuel is fed to the JFS, the ignition fires and it starts
the JFS throttles up, producing about 200 horsepower
when the JFS is running full tilt, the engine shaft's clutch engages
the turning shaft rotates the main engine until it reaches start speed
once the main engine reaches a particular speed, the JFS is declutched from the gearbox and shuts down
the spinning engine brings the generators and hydraulics online, recharging the pressure in the accumulators for the next time it needs to start

In some respects, this is similar to a bulldozer or other heavy equipment.  Many of them have a small "pony motor" with an electric starter, which then produces the power/torque combination necessary to start the main engine.  Once the main engine is running, the pony motor declutches and shuts off.
Other, older aircraft, such as the F-100 Super Sabre, used a small, explosive charge (start cartridge, essentially a large blank).  You'd install it into the engine.  The pilot would electrically fire it into the exhaust turbine, which would spin up the engine enough (you hoped) to get the whole thing to start speed, then engage the fuel and ignition systems.  If the cart failed to fire properly, you had to wait a certain period of time for it to cool off before you could remove it.  You wouldn't want it going off on the ramp (possible presence of fuel vapors) or in your hands (ouch!).

When I see the picture of a JFS, a large watermelon doesn't exactly come into mind.

Admittedly, it doesn't LOOK LIKE a large watermelon.  But, for purposes of scale, that's about how large it is.  And I have picked one up and carried it, much like a large watermelon.

A 200-horsepower watermelon!

The pony engine on tractors etc. also is water cooled and shares the water jacket with the main engine.  So it preheats the main engine also.

I guess now it is my turn to add exotic ways to start a jet engine.
The Jumo 004 of the Me-262 and Ar-234 had a small piston engine in the central spike which was electrically started, but also could be hand-started by a mechanic. Did you ever note the hole in the middle of the spike, and the ring stuck in it (see the left picture below)?

The ring was attached to a string which could be used to start the piston engine (Riedel starter engine, right picture). This could spool up the engine by itself up to 2000 RPM. At 500 - 1000 RPM the pilot would switch on the ignition. Now the starting combustion helped to spool up the engine further, until at 5000 RPM when the piston engine would decouple. Then the pilot could advance throttles slowly to 8700 RPM, the maximum.
What could go wrong? A lot:

The piston engine would not decouple.

Insufficient fuel flow, so the engine would not accelerate. In cold temperatures, the fuel pumps had to be switched on, but only up
to 3000 RPM.

The ignition would not start soon enough. Then the rotating engine would supply fuel to the combustion chamber, where it would collect and be blown into the rear engine fairing. When the combustion started, the collected fuel would burn as well, resulting in an engine fire.

The pilot would advance the throttles too quickly. This would supply too much fuel to the combustion chamber and heat up the turbine above its limit. Only with the higher airflow near the design
speed could the throttle be firewalled.  -

The other starting procedure is that of the J-58s in the SR-71, which was remarkably similar. Only that the starter engine was a lot bigger and not part of the engine: It was a custom design of two tuned Buick 455 V-8s, coupled together on a cart and driving a vertical drive shaft that was driving the engine from a port under the engine nacelle. To start ignition, a special compound called triethylborane was injected, because the cold JP-7 (the special fuel for J-58s) would not ignite by itself. Once the engine could sustain itself, the cart was disengaged.

The Buick engines would frequently fail and were later replaced by two Chevy big blocks. Also, the engines could be started with pressurized air. See this link for more.
What else could go wrong? That list is quite similar as well ...

So, you start the Jumo 004 with a pull, just like your lawn mower? That's fantastic!

@FreeMan: Yes, the Riedel engine in the Jumo was not too different from a lawn mower engine.

For a very basic turbine startup, no FADEC or computer control, here's a video of a Mosquito XET turbine helicopter being started.
Throttle at 50%, then cut back to idle position.
Engage starter motor.
At 17% RPM, turn on the igniter.
At 30% RPM, turn on the fuel... the rumble indicates that the burner can has ignited.
At 50%, turn off the starter.
At 70%, turn off the igniter.
At 100%, it's ready to fly.
The Mosquito uses a 90 hp Solar gas turbine originally made for backup generators, which is one reason why the whole package is (relatively) inexpensive: around $65k-70k for a turbine powered helicopter.
Some assembly required...

There is also the shaft drive iteration of starting engines. As a time served tech on Tornado's I cant begin to tell you how frustrating this system is!
An APU operates a gearbox, which has a generator, a hydraulic pump, and an output shaft to the engine to start it.
 Its also coupled to another gearbox on no2 engine through a shaft, which also has a hydraulic pump, generator and output shaft. So the APU or either engine can run either, or both gearboxes and start either or both engines!
Feel safe sharing the operation of it now the aircraft is out of service in the UK anyway ;)
http://www.tornado-data.com/Tid/systems/common/secondary_power_system.htm


text:
For taking-off and landing in a DHC-7, what elements can be used in writing a novel?
What cockpit elements can be used in writing a novel for taking off and landing in a de Havilland Canada Dash 7 from a short runway in Rothera, Antarctica? Including the names of those switches overhead, and the throttle and the steering stick/wheel?

You typed Dash 7 in the question but tagged it with Dash 8. Which type are you referring to?

@kevin There is no Dash-7 tag, probably used that one because it was close.

Yes, that's why I had to use that Dash-8, it wouldn't let me. It's a de Havilland Canada DASH 7.

Taking off on a short runway in Antarctica but in good conditions.

If you want to know what switches are used when, easiest thing to do is to find all the checklists for the Dash 7.

I'm trying to do that but they're not as basic as what do you call the steering wheel, because I know it won't be the steering wheel!

Sorry, don't understand this forum very well but thank you to whoever adjusted the tags for me as well as editing my title etc.

Not specific to the DHC-7, but for general "how do you even land a plane?" research, consider watching some actual-aviation shows. Air Crash Investigation isn't bad IMO for the in-cockpit (and in-simulator) scenes, including the terminology used by the pilots. There's also a bunch of videos on Youtube of actual pilots landing actual planes, everything from a tiny GA airplane like a two-seat Cessna up to big jets that fly intercontinental routes with hundreds of passengers. Pay attention to how the flight crew members interact, what they are focusing on, in what order things are done, etc.


text:
Is "hacking small planes" a separate or larger problem from/than general small-aircraft security?
Apparently the DHS has issued a "hacking alert" for small aircraft:

The alert warns if a hacker gained physical access to certain small planes, they could attach a common device to its wiring. This could provide false flight data including altitude and airspeed, as well as possible access to the autopilot system, potentially leaving a pilot unable to tell whether the reading was accurate and potentially lose control of the aircraft.

This involves physical access to the aircraft, which the FAA said "is unlikely."
Is this particularly different from other attacks not involving "computer hacking" that could be mounted given physical access to an aircraft? For example, is this in some way much less likely to be caught via a pre-flight inspection than any other attack on avionics or other equipment, or much harder to mitigate in flight?
Or is this just more Security Theatre from Homeland Security, raising what's essentially already a known mode of failure (potentially from reasons other than a "hacking attack") for which the aviation community already has procedures in place to mitigate it and continues to monitor and improve these procedures as technology changes over time?

What percentage of small planes even HAVE autopilots?  Or any electronic instrument other than the transponder (which produces no output that he pilot can see)?

@jamesqf I can't give percentages, but from articles and advertising it appears to me that full glass cockpits are standard in a substantial percentage of new GA aircraft in the last dozen or so years, and even in older aircraft adding GPS (which is by definition computer-based) seems to be a popular option. In my exploration of 70s-90s era electronics and computing, I've seen plenty of projects related to embedded systems in aircraft using microcontrollers, which are mainly software-driven. (And it's not hard to pull a ROM from one of those and replace it with another.)

But new GA airplanes are a small fraction of the fleet.

@jamesqf Ah, ok, I understand now. It's just a small fraction of the fleet that could be affected by this, so it's safe to ignore.

It sounds like just another theoretical scenario.
If that "hacker" wanted to do harm, he could also loosen a few bolts, disrupt the engine timing, plant a bomb, you name it.
Sure, in today's world of aircraft loaded with electronic systems the operators have little knowledge of and trust implicitly, it's quite possible that something that deliberately feeds wrong data to those systems goes unnoticed than disrupting the cylinder timing or mixture control cables would have in the past, but it's still a highly remote possibility.
What it sounds to like me is someone taking the scare of "hackers" breaking into your car and taking over the engine electronics to the next level.

Do pilots really "trust implicitly" any of their systems? Given the extensive procedures for dealing with, e.g. radio and transponder failures, it seems clear to me that it's never trusted that those will always be working correctly. My impression is that in aviation pretty much everything has been analyzed for failure modes and mitigation put in place in line with cost-benefit ratios far exceeding what we find acceptable in day to day life. Sure, there's not a huge amount of mitigation for "the tail fell off the aircraft," but a lot of work at least goes into making that very unlikely.

@CurtJ.Sampson they do. And with the electronics they ever more often have to as the electronics are often the only way to get information about aircraft systems at all. Take pitot tubes. Many modern aircraft no longer have vacuum instruments, instead there is a system that converts the incoming pressure into an electrical signal which gets turned into a number by a microprocessor. That gets sent to the pressure gauges. If you place something in the aircraft that intercepts and replaces that value the pilot has no way of knowing.

Getting into the wiring on a small or medium or large aircraft is not easy. Need to get past airport security, possibly into a locked hangar, then into the plane (pretty visible if just tied down outside too), then up under the panel to trace out bundled, unlabelled wiring with no schematics. Not too easy.

Pitot tube is not a vacuum instrument. It can be digitized and displayed (on a Garmin G5 for example) but also shows up on the airspeed indicator, which is strictly air pressure against a spring.

@Curt J. Sampson: Perhaps other people fly differently than I do, but as long as I'm flying VFR (and that's really all I do), I really don't use the instruments all that much at all.

I use the GPS all the time to keep me on track vs wandering around all over. Burn less fuel getting from point to point that way. @ $45-50 an hour, and with lots of controlled airspace around, above, and below me, position is important to know. Altitude when flying cross country is important too, need to hold your 500' or 1000' level to avoid ATC speaking to you.

@CrossRoads yes, but that's exactly the scenario the advisory/warning describes, someone being able to get into the wiring of an aircraft and make it so it gives false signals to the electronics (or getting into the computers and installing malware on them). Not easy to do unless the airfield lacks security, which many GA fields do (was at one in Canada last year where the only security consisted of simple locks on the hangar doors, locks of the kind you get at your local DIY store which is probably exactly where they came from).

Ok, I've got 6 boxes plus WAAS antenna feed all talking together, and I bet they all have surface mount parts these days. What do you think the odds are of someone manipulating the software on any one of them, and it continuing to pass its startup tests and then still talk correctly to the others without error messages, and talk correctly to ATC the next random time I departed, and neither I or ATC not noticing something funny? Sounds very unlikely.

@CrossRoads tell that to the TSA, they're the ones floating the alarm... All I do is show that it is indeed theoretically possible, even if highly unlikely.

TSA? The people screening us for entry into planes? They're hardly technical types.

@CrossRoads yes, DHS, of which TSA is a part. FAA countered that such a scenario is "unlikely". Which is what we determined as well.

I was thinking driving in that my old Northstar GPS had to be turned in to factory for software updates.  Or the soldered-in battery. I had to remove it from the panel to change the PCMCIA (2 Megabyte card) with approach info. New ones are software updated via uSD card, and an AP has to sign off on it. Database updates are done by users via USB stick.  So to change programs, a corrupter would need the source code of a box - the OEMs (Garmin, Avidyne, etc.) don't release that, they only distribute the executable code, so it would take someone with inside access to develop a change.

@CrossRoads "...a corrupter would need the source code of a box..." I'm sorry this comes across as harsh, but you seriously have no clue about hacking. More often than not, the object-code-modders have better tools for finding vulnerabilities than the developers writing the source code.

It's still quite the technical challenge to try to load a compromised binary, as the hardware in almost every LRU is going to be different.

@CurtJ.Sampson indeed. I work in software engineering and have helped in developing binary patches to software to which we lacked the source code. Kernel debugging is as much fun as it can be frustrating. Does require access to the actual operating system running on actual hardware to accomplish of course.

CNN's take on the DHS announcement:
https://www.cnn.com/2019/07/30/politics/airplane-hacking-risk/index.html

Washington (CNN)  Physical hacking of airplane electrical systems is a realistic possibility that could bring down aircraft, the Department of Homeland Security warned Tuesday.
The government said researchers at a cybersecurity firm identified ways an attacker could cause aircraft displays to show false "engine telemetry readings, compass and attitude data, altitude, airspeeds, and angle of attack."
"The researchers have further outlined that a pilot relying on instrument readings would be unable to distinguish between false and legitimate readings, which could result in loss of control of the affected aircraft," wrote the Cybersecurity and Infrastructure Security Agency, which is part of DHS.
Rapid7, the research firm, said it investigated the risks with small aircraft rather than larger commercial planes.
Hackers must have physical access to the aircraft, the company said. The attack involves physically plugging a malicious device into the electronics systems that increasingly control the displays and functions of modern aircraft.
Similar -- but more secure -- technology is used in cars. The Rapid7 researchers attributed that difference to the fact that "even small, personal aircraft are rarely parked in unmonitored, open areas like open parking lots or public streets," as is the case for cars.
The DHS notice recommended that "aircraft owners restrict access to planes to the best of their abilities" and that manufacturers review the security of their electronic systems, and consider using protections similar to those used in cars.
Neither the company nor DHS identified specific aircraft or manufacturers.

CBS's article that was called out, with a picture of Cessna 172s and other planes to lead off:

Washington — The Department of Homeland Security (DHS) issued a rare security alert Tuesday for small planes. The alert warns if a hacker gained physical access to certain small planes, they could attach a common device to its wiring. This could provide false flight data including altitude and airspeed, as well as possible access to the autopilot system, potentially leaving a pilot unable to tell whether the reading was accurate and potentially lose control of the aircraft.
"We could do basically anything the plane can do, pretty much anything the pilot could do by himself," said Tod Beardsley, director of research at cyber security firm Rapid7.
In a laboratory setting, researchers at Rapid7 were able to hack what's essentially a plane's electronic central nervous system, allowing them to send the erroneous commands. "I do think it is something of a wake up call. This is something that needs to be taken seriously now rather than later, like after the disaster," Beardsley said.
Last year, Homeland Security researchers, who successfully hacked a 757 while it was parked, warned it's only "a matter of time before a cybersecurity breach on an airline occurs." DHS is now urging small plane makers to study the cyber security protections car makers added after hackers were able to exploit similar technology on the roads.
The Federal Aviation Administration tells CBS News a "scenario that involves unrestricted physical access is unlikely" but these findings are "an important reminder to remain vigilant." Rapid7 said it notified the company that built those two systems, but their findings prompted DHS to urge pilots to secure their aircraft and for manufacturers to take a look at options to better secure these systems.

I don't see this happening to anything but the large airplanes that have automated systems.
Most smaller prop planes have no automated controls except for the autopilot - and that is easily turned off by flipping a switch, or pulling the autopilot circuit breaker.  Nothing that bad data on a display is going to override.  Engine controls are manual only, so there will be no planes falling out of the sky.  There are old-school backups for most of the rest - altimeter fed from a hose from the static ports, attitude from backup artificial horizon and turn coordinator, compass heading from a whiskey compass, airspeed from a hose from the pitot tube, angle of attack from the artificial horizon again.  Engine readings, one can look at the engine controls and see there is nothing odd set: throttle, RPM, mixture.
DHS needs to be more responsible in describing the plane types that could be affected.

Main affected aircraft would be those with G1000 cockpits and similar. There digital displays replace all the flight instruments, often there are no analog backups at all. Only indication the pilot would have if the engine readings are off is that the engine sounds different from what the readings tell him it should.

Not true at all re:analog backups. Look at the Avionics presented here:  https://cessna.txtav.com/en/piston/cessna-skyhawk#_model-avionics  Airspeed indicator, Artificial Horizon, Altimeter, and compass on top of the dash are still old school dials.  Engine control is still manual knobs, altho engine info display has gone digital (I have done the same, with JPI EDM900 to show all engine parameters).  Similar can be seen in the other models.

@jwenting: Aren't analog backups a certification requirement?

@Vikki nope, certainly not for most light aircraft and especially not for experimentals and home builds.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What Kind of airfoil configuration is preferred for an hyper sonic plane?
I am trying to design a hyper sonic aircraft in line with X-15 for my Aircraft design project. This requires me to figure out the preferred airfoil shape for the wing.

If there was one optimal shape for all situations, this would be easy to answer, as it's written it's impossible to answer.

I'm not sure this question has to be on hold, Wikipedia article provides a seemingly valid answer to this broad question: "Supersonic airfoils generally have a thin section formed of either angled planes or opposed arcs (called "double wedge airfoils" and "biconvex airfoils" respectively), with very sharp leading and trailing edges".

At hypersonic speed the local drag grows with the inclination of the local surface against the free stream. Thus, the best shape is a thin wedge. Drag will scale with the cube of the thickness ratio of the wedge.
Depending on the maximum tolerable heat load, you should choose a pointed leading edge (highest heat load, smallest drag) or a blunt leading edge, where the separated shock will reduce local heat loads, albeit at the price of much higher drag.
Ideally, past its maximum thickness the airfoil tapers down towards a thin trailing edge, and again a wedge shape will cause the least amount of drag.
In the picture below (source) you can barely see the tip airfoil of the X-15:

It used a simple, blunt wedge at the vertical tails because the vacuum at the rear face provided additional directional stability. However, for lowest drag, a double wedge of the smallest thickness possible should be used.


text:
Where can you find wind tunnel visualization data online?
I was wondering if there was a repository of flow visualization results somewhere online, preferably of wing sections categorised in some manner.

The closest you will find to a comprehensive, freely accessible source of data on this is the NASA Technical Reports Server. It is a search tool that gives you access to many NACA reports on various airfoils.

You can check the ERCOFTAC Classic database collection at: http://cfd.mace.manchester.ac.uk/ercoftac/index.html
It is a comprehensive database used by fluid-dynamicist for model validation, and it contains many flow visualizations including a couple of wing test cases. Experimental data are free to download.


text:
"Selected/commanded," "indicated," what's the third word?
Aviation terminology, in my understanding of it, makes a useful distinction between something being "selected" or "commanded" and "indicated."
For example, if I flip the lever for landing gear from "up" to "down," I've "selected" landing gear down or "commanded" it to be down. This doesn't, of course, mean that the gear is actually down and locked into position so that it's in a suitable state for landing.
Thus there will often be a light or other indicator connected to sensors that determines through means other than the lever position that the landing gear is in the desired state; when this light goes on (or is the correct colour or whatever) it's said that the gear is "indicated" down, giving me some confirmation that the command has been correctly executed.
This, too, doesn't mean that the landing gear actually is in the commanded state; there could be an error or malfunction in the system such that the landing gear is indicated in that state but is not, or vice versa.
There is surely a third word used to say what the true state is (even if this state may not be easily detectable by the pilots). What is this word?
(If I've got my terminology wrong here, or there are multiple terms used for these things, please correct me in your answer.)

Perhaps "confirmed"?

The word is "actual".
"Selection" refers to the interface between the human and the hardware. Finger or hand, and switch, or selector handle.  A selection is a human choice.
A "Command" refers to the signal resulting from the selection, within the hardware or software system, that initiates the actions to get the final result .  Switch moved to position A, results in signals to a controller that tell the controller to do this or that, to get the result intended by the human.  The controller received a command from the switch, when the pilot made the selection.
"Indication" tells the human that the final result was what he/she wanted.
The true state resulting is the "actual" state.  Hopefully it matches the indication, or if the indication circuit is lying to you because of a malfunction, it doesn't.
So, if gear is only halfway down even though you have a down and locked indication, you'd say halfway down is its actual position after visually looking at it yourself, and Houston, we have a problem.

Depending on design, an "indication" can be based off either the selected or the actual state. "Talkback" is a useful word sometimes used for the latter. Gear indicators are usually talkbacks. (Of course talkbacks can still fail in a way that they no longer reflect the actual state!)

Was in the aerospace business 40+ years and have never heard the word "talkback". That sounds like something from another industry.  You just say "downlock indication", "in transit" indication, etc.  Or maybe it's a Boeing thing?

It's not specific to gear indicators. From poking around in the Google, it seems this term may be mostly limited to spacecraft.

+1 for actual.  Another sub-type of indicated + actual is verified

"Verified" or "confirmed" refers to something the pilot has done. The actual state is the true state, whether or not the verification/confirmation (whatever that entails for a given system) has been performed.


text:
What is this aircraft-like object at Sea-Tac Airport?
Looking at the Seattle-Tacoma International Airport on Google maps, I noticed at 47°27'47.8"N 122°18'15.5"W something that looks like a jet aircraft, but with folded wings and a tiny empennage. It's just left of centre in the image below; I've included a couple of nearby aircraft at the top for scale and comparison.

What is this object? It looks superficially like an aircraft, but I've never seen a non-carrier jet aircraft that folded so much of its wing and the empennage is surely far too small to provide sufficient stability and control for flight.
Could it perhaps be some sort of training platform for ground crew (regular or emergency), with folding simulated wings to make it easier to store?

Congratulations on achieving 1000 reputation and unlocking new privileges!

This is definitely not real aircraft. Most probably, it is for firefighting/deicing practice. At London Heathrow, they have a model of a 747 with three engines, and it is intended for ground training purposes.


text:
What is the OUTBD PFD setting on the 737 NG MAIN PANEL DUs switch?
Each pilot on a Boeing 737 NG has a five-position switch labeled MAIN PANEL DUs that selects what appears on the two main display unit panels in front of her. Here's a picture of the left pilot's switch:

Most of this seems pretty obvious to me: NORM gives you the PFD (Pilot Flight Display with attitude, altitude, speed, etc. indicators) on the outboard DU and the navigational display on the inboard DU; and the three INBD settings change the inboard display to show the engine primary display (normally on the centre console upper panel), PFD, or multi-function display (normally on the centre console lower panel).
What does the OUTBD PFD setting do? In NORM mode the PFD is already on the outboard DU, so it sounds as if this wouldn't change anything.

It blanks the inboard display & forces the PFD to its normal place. It would take a combination of failures for this to be needed, but if the system sensed the wrong display as having failed & was trying to put the PFD on the inboard (actually failed) screen, then this would get it back to the (actually working) outboard screen. A really, really unlikely case.
In 20 years, I don't recall ever needing to use these switches for an actual failure in the aircraft. Maybe once or twice in the simulator.

Ah! I had no idea that automatic systems might switch the displays around themselves.

@cjs  They do, although not every failure is something they would necessarily detect. Something like a failed power source might be easier to detect than some types of damage to the display screen, for instance. The system will try to take care of you, but if it's not actually being helpful, these switches allow the pilot to override it.


text:
What sort of issues related to extreme heat (115°F, 46°C plus) would prevent takeoffs or landings?
I recently flew into and out of McCarran Airport Las Vegas (LAS) and a local told me that they never report the air temperature at the airport above 115°F (46°C) because the FAA does not allow landings above that temperature.
What sort of issues do large passenger jets (or small for that matter) have to deal with in high temperature environments either taking off or landing?

The "local" was simply ill-informed.  The reporting of weather conditions is critical to flight safety.  Imagine the legal liability if they deliberately under-reported the temperature and an accident occurred because of that.  46c would also be extreme for Las Vegas so the fact the METARs report a temperature lower than this is, well, to be expected.

The FAA doesn't allow any aircraft operations if air pressure is extremely high, that might have been what your local guy was talking about.

@Pondlife:  high pressure is not a problem.  In fact, high air pressure is good for flight.  High density altitude is a problem, and high density altitude means low pressure.

Heat can cause delays or cancellations but it's generally due to each aircraft's specific performance limitations, not a hard limit set by the FAA.

@abelenky 14 CFR 91.144: no flight operations are allowed if air pressure exceeds 31.00". In reality, if it happens they issue a NOTAM and people can keep flying (see the AIM 7-2-2). It's unusual, but it is an actual regulation, whereas there's none that I can see related to temperature. I was wondering if the local guy was just half-remembering it when he said that the FAA doesn't allow landings in high temperatures.

91.144 is because altimeters are not accurate at such high pressure.   Most cannot be adjusted that much.  However, aircraft performance is fine.  I don't think I'd be concerned about an all VFR flight, where precise altitude is not important, at an altimeter of > 31.00.  (source: http://studentpilot.com/interact/forum/showthread.php?22278-FAR-91-144-Abnormally-high-barometric-pressure)

@pondlife in fact, I believe the entire point of that regulation is to make sure that a NOTAM has been issued... the idea is to make sure everyone's aware, rather than to actually prevent operation

@JonStory That's probably right, but the details of 91.144 aren't important here; I was simply speculating that the local guy was confusing a regulation on air pressure with one on air temperature.

Aircraft performance is a many-faceted topic. Insofar as it being so hot that operation is prohibited, a couple of things come to mind.
Aircraft performance figures are supplied by the aircraft manufacturer, typically as tables, up to a certain temperature. Since operators are required to use those tables for determining the aircraft's ability to safely takeoff, if the temp is above the max temp in the table, there is then no available, tested data to do that and legal operation would stop. I just checked the performance data for a 747-200 with P&W JT9D-7Q engines, and the tables top out at 123 °F (50.5 °C).
Another factor is the maximum fuel temperature. For 747-100/200 aircraft, that temp was 54 °C as I remember, about 129 °F.
Now I can't speak to today's operating environment since I retired in 1999, but in the 1990s fudging or ignoring temperature requirements was common in some third-world countries. For example, if we were in the countries around the southern end of the Arabian Gulf (Iran called it the Persian Gulf) in the summer and the aircraft was fueled and had set for a day or two in the sun with mid day temps in the shade of 120 °F (49 °C) or more, you knew the fuel temp might be a bit above 54 °C, but nobody ever checked it. What you did do was account for that possibility in your planning. You might, for example, choose to do a full power takeoff rather than the reduced power takeoff that you might otherwise have used.
I personally was never aware of any mis-reporting of the temperature in the U.S., but in the 1980s when I was flying Metroliners for a commuter, there was one airport that as a matter of course typically didn't report a visibility less than the minimum required to start an approach when a commuter flight was due in. Once you reported the outer marker inbound (non-radar environment back then), they told you what the actual RVR was. Since you were inside the outer marker, it was legal to continue the approach, and we always did, and we always got in. That made everybody happy: the airline, the tower, and the local chamber of commerce.

Offtopic, but the Gulf is actually called Persian Gulf internationally. Only some Arabic countries refer to it as Arabian Gulf.

@Zeus Offtopic, but most Americans and I think most of  the British largely dropped the "Persian" from the Gulf when the Shah joined the Nazis in WWII and changed the name of the country from "Persia" to "Iran" an alternate spelling of "Aryan".  They weren't going to call it the "Aryan Gulf" I've seen maps from the late 40s where the name had already switched to Arabian when pre-WWII maps show it as Persian.

I think you have a temperature unit conversion error. Google says 123F = 50.5C (not 55.5C) and 129F = 53.9C (correct), but I won't propose an edit because I have no idea which figures (Celsius or Fahrenheit) are the ones that would be specified by the manufacturer.

@TechZen Off-topic and largely incorrect. The Iranians have always called their country something like "Iran", and it was in 1935 that they started asking other people to call it that, too -- the idea of calling it "Persia" come from the Greeks. See Wikipedia. Although the words "Iran" and "Aryan" do come from the same root, the name change was not because of WWII and there would be no reason to start calling the Gulf the "Aryan Gulf" rather than the "Iranian Gulf".

@MichaelKjörling You are correct, and I will edit the answer. However, I would like to point out that the mistake was not mine. My answer originally just said 123F, and someone else edited in the (55.5C). I'm 77, and, frankly, am prone to making errors that I would would never have made when I was young, but that error I did not make. LOL

@Terry Fair enough; I didn't check the revision history before commenting. :)

@TechZen: No, most Americans still call it the Persian Gulf.  Source: am an American.

Would be great and much appreciated if you could elaborate in this answer regarding the maximum fuel temperature. With four straight record-setting days with respect to global average temperatures, localized heat waves have been grounding flights, and a friend of a friend reports that their jet was grounded due to this max temp limit, but it's unclear to me exactly why this limit would exist. I've speculated -- things like maybe for a flight thar requires a full fuel load, the fuel density would be too low to allow enough fuel to be loaded, or maybe the fuel metering system is based on ...

... volume and can't compensate if the density is too far outside a nominal range -- but I'd really love to learn from a reliable source what the actual reason is, and haven't been able to find even any real mention of the issue, never mind an answer.

Tires...
... at least that's were it started. My grandfather was a pilot starting back in the 30s, and got to see the whole technology develop. Blowing tires was big problem for big aircraft from WWII bomber up to airliners in 70s. Used to see actual blown tires at the end of runways pretty often. When I was kid, in the late 70s, a taxi at Love field in Dallas, Tx got hit by a fragment of blown tire in June when temps were already over 100F.
Compared to ground vehicle tires, aircraft tires are relatively thin walled and balloon like because

they have deal with a lot pressure variation
they have to absorb shock
they have to roll at speeds which in a ground vehicle would qualify them as a race car, which generates a lot of heat
they need large contact patches, but still need to conserve weight

However, balloon-like tires are more sensitive to expansion due to heat. At 115°F (43°C), tires of planes on the tarmac taking off will be near the edge of their expansion spec already. In landing planes, the tires will go from sub-zero to near overheating even before the tires hit the tarmac.
I think military transports have some sort of cooling system to allow all weather operations I've seen C-130 variants land in hot weather and then something that looks like a fire extinguisher exhaust comes out of port in the back of the wheel assembly i.e. it's not burning rubber. It's probably some tech like nitrogen pressurized fuel tanks, allows landings in any weather but to expensive for civilian aircraft. But I never looked into it so I could be wrong.

If the temperature is hot enough, the runway can buckle. The asphalt expands as it heats and eventually expands so much that it starts folding up. This has caused runways to close (in the US and elsewhere) but there doesn't seem to be a specific regulation, it seems to be handled with runway inspections and closing the runway whenever a problem is found.

very valid point, but generally only impacts asphalt runways, not concrete or grass. Well, it has to get really hot before concrete begins to soften, and when that happens, I think we have other concerns. :)

Welllll, it does also impact the McMurdoch Ice Runway: https://en.wikipedia.org/wiki/Ice_Runway Although I was specifically thinking of the 2012 DCA melting, which was asphalt: https://www.washingtonpost.com/local/crime/2012/07/08/gJQAZgG9UW_story.html Probably more of a concern for older runways in 4-season climates, where they were originally engineered to withstand seasonal cold but then have a sudden heat wave.

It isn't necessarily the airport operator that limits takeoffs and landings at such hot days but the manufacturer of the aircraft.  All Part 25 certificated airplanes require performance data for all weights, altitudes and temperatures they operate in.  Most likely the aircraft manufacturer has chosen to limit takeoffs and landings on such hot days because they chose not to publish the applicable data.

§25.105(a) The takeoff speeds prescribed by §25.107, the accelerate-stop distance prescribed by §25.109, the takeoff path prescribed by §25.111, the takeoff distance and takeoff run prescribed by §25.113, and the net takeoff flight path prescribed by §25.115, must be determined in the selected configuration for takeoff at each weight, altitude, and ambient temperature within the operational limits selected by the applicant—

It's because of air density.  The density of the air isn't constant and varies with elevation above sea level and air temperature.
$$ F_{lift} = C_{lift} \times \dfrac{1}{2} \times \rho \times V^2 \times A $$
… where $A$ is the wing area, $V$ is the velocity and $\rho$ is the density of the air.  If the air isn't dense enough the aircraft won't be able to take off if it can't generate enough lift at the end of the takeoff run or will have to land at a speed that is too high to keep it from stalling.

Density altitude is the altitude relative to the standard atmosphere conditions (ISA) at which the air density would be equal to the indicated air density at the place of observation. In other words, density altitude is air density given as a height above mean sea level. "Density altitude" can also be considered to be the pressure altitude adjusted for non-standard temperature.
Both an increase in temperature, decrease in atmospheric pressure, and, to a much lesser degree, increase in humidity will cause an increase in density altitude. In hot and humid conditions, the density altitude at a particular location may be significantly higher than the true altitude.
In aviation, the density altitude is used to assess the aircraft's aerodynamic performance under certain weather conditions. The lift generated by the aircraft's airfoils and the relation between indicated and true airspeed are also subject to air density changes. Furthermore, the power delivered by the aircraft's engine is affected by the air density and air composition.

Not sure I buy that given that dealing with changes in air density with altitude is a basic prerequisites for flying in the first place. Back in the pre-WWII days, low power aircraft had to struggle to get off the ground from high-altitude fields, but today when even a civilian airliners has enough power to almost stand on its tail, I don't think its much of an issue.

Aircraft performance is directly affected by the ambient atmospheric conditions.  Specifically, both lift from wings (or rotors), engine power and thrust in the case of jet engines is proportional to the density of the air.  Dense, cold  air creates a lot more lift for a given true airspeed than does hot, thin air and engines produce more power in denser and/or thrust in denser air than they do in thinner air.
Air density is a function of both atmospheric pressure and temperature using the ideal gas law
p = drT
d = p/(rT)
where p is the ambient air pressure, d is the density of the air and T is the ambient temperature.  r is the ideal gas constant.
So the density is directly proportional to the temperature of the air and inversely proportional to the temperature.
In aviation, the pressure is a factor of both field elevation and ambient atmospheric pressure in the area.  The high up you go from sea level, the less dense the air gets for any given temperature.
Now what all this means for a pilot, is that, since air density affects both lift and thrust, the performance characteristics of the aircraft are going to be much more lethargic on a hot day.  This is compounded by the field elevation and flying altitude as well as pressure deviation from standard atmospheric condition.
These 'hot and high' conditions can be extremely hazardous and a lot of good pilots have been killed because they did not factor this in with performance calculations related to runway length needed for takeoff and climb gradient to clear obstacles.  The runway needed to accelerate to Vr and clear an obstacle may be as much as 200-300% greater than required for a sea level takeoff, and the maximum rate of climb at Vy on a hot day may be halved compared to cold, sea level operations.  This is particularly perilous for bush flying and operations over mountainous terrain.
Below is a link to a video of a Stinson 108 light aircraft departing a grass strip near Stanley, Idaho with a 6000+ ft field elevation on an 85° summer day.  The aircraft was also loaded with four people at over gross weight.  WARNING: graphic content; if airplane crashes upset you, do not watch.

The good news is that everyone aboard was lucky and walked away from the accident; most people involved with this kind of emergency aren't so lucky.
Performance data for aircraft is published by the manufacturer in the required pilot's operating handbook (POH) and performance calculations should be performed prior to all flight operations.  Ambient atmospheric conditions are available to the pilot either in the form of ATIS/AWOS/ASOS broadcasts or using portable weather monitoring equipment. ATIS broadcasts make the process easier by broadcasting a density altitude for the field which is equivalent to a field altitude if the ambient air were at standard temperature and pressure of 15° Celsius and 29.92 inches of Mercury.
I can't speak for McCarran International's (LAS/KLAS) operating procedures, but I suspect they do not allow operations above 40° C both because the runways there would be insufficient for launching and recovering heavy transports in those conditions and, in general, aircraft manufacturers do not specify performance data for temperatures beyond 45°C / 110°F.


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What should I study to understand basics of aircraft construction?
I'm trying to undestand principles of construction of fixed-wing subsonic aircraft.
I have already read explanation on things such as Neutral Point, Static Margin, etc, but I'd like to know how they are linked together, what are typical values they have and why, what are pros and cons of different schemes, and so on. Some of this can be found here, but I think it would be more sound to learn basics first, and start looking for details afterwards.
A good example of literature (or Internet resource) I seek can be "The Jet Engine" book by Rolls Royce. It explains nature of processes while don't digging too much in formulas.
Thank you!

Thank you for putting this question on hold without any explanation what exactly is too broad and how can I correct it. Glad there is one who found few minutes to advise.

I have seen many universities that they suggest Anderson's Introduction to Flight for the first course in aerospace engineering. This book contains fundamental aspects of the aeronautics.
Regarding construction principles, a good knowledge of aerodynamic, propulsion, structure and flight dynamics is required before getting into aircraft design. Meanwhile, as the first step you can familiarize yourself with the anatomy of the airplane with Stinton's Anatomy of the Aeroplane, and then going to aircraft design books such as Roskam's Airplane Design Books or Torenbeek's Design book.

Thank you very much for your advice. It seems exactly what I need!


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Are Helicopters allowed near busy airports?
Are Helicopters allowed near busy airports or any airports at all and if not why not

How would helicopters not be allowed near any airports at all?

The West Midlands Police helicopter is based at the busy Birmingham International.....

Define "busy", "allowed", and "near".  Certainly there are helicopters based at the local comercial field (Reno) which is moderately busy, medical helicopters regularly land at the hospital that's maybe a mile west of the field (crossing the approach if they're coming from the east), helicopters regularly fly between there and South Lake Tahoe, military helicopters frequently land there, or transit the airspace from the Stead National Guard base...

Like all other aircraft, they are not allowed to fly over the airport or cross into arrival and departure airspace without clearance. It is the task of the air traffic controls to deconflict the aircraft appropriately.

Yes, helicopters are allowed near busy (and any other) airports, because why not?
Ok, seriously: helicopters in general are just as able aircraft as ones with fixed wings. General limitations for use of airspace may be in effect depending on airspace categories around aiports, but to my knowledge there are no airports in the world that would specifially prohibit operation of helicopters in their airspace.
Some private airfields may have restrictions for helicopters and other aircraft due to noise, but I'm guessing this is not what you are asking?


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How are the dimensions of the empennage determined for jetliners?
How is the height of the vertical and span of the horizontal stabilizers determined for different turbofan aircraft like the Airbus A380 and the Boeing 747-8?
What are the maximum forces (in kilograms or Newtons) acting on both the horizontal and vertical stabilizers for aircraft like the Airbus A380 or the Boeing 747-8?

That's practically four separate questions--pitching and yawing moments have different sets of causes. Consider https://aviation.stackexchange.com/questions/45996/is-there-a-formula-to-calculate-how-big-a-ventral-fin-should-be (which has an answer addressing one of your four questions: "it's complicated").


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What is the name of the system that replaced stairs for disembarking aircraft?
Back in the day, or in old airports, or airports that have not upgraded their systems, passengers disembarked in aircraft using stairs, through rain and or snow and extreme weather, sometimes having to delay disembarking because of bad weather in turn delaying themselves, the airport and the aircraft schedule. Today there is a new weather proof system where passenger disembark the aircraft and find themselves in the air conditioned airport, devoid of all the unfavorable extremes, devoid of delays.  What is the name of this system and when was it introduced?

You're probably thinking of jet bridge, which is basically a movable tunnel connecting the terminal and the aircraft.


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Does the 747-8 / A380 nose-wheel encounter more weight during braking?
Given that the nose wheels of both the Boeing 747-8 and Airbus A380, as is true for almost all aircraft, are not equipped with brakes, do they encounter more weight during braking?
If so, how much more at maximum safe landing weight?
If they were equipped with brakes, would they encounter more weight during braking and what other effects would ensue?

Braking puts more weight on the front wheel(s) of any vehicle - physics demands it.

@FreeMan Beat me to it, how much more depends on how much it weighs, how hard it is braking, and the distance between the main wheels and the nose wheel.

@FreeMan be careful with the word any. What if the the vehicle's centre of gravity is below the wheels?  What if aerodynamic braking is used? Or speed brakes?

@DeltaLima good points. I would postulate, though, that weight will still shift forward for the suspended trolley, though that might produce a lifting force on the front wheels. Popping a chute behind the plane would cause a different weight shift. Original statement valid for any conventional, wheel-braked vehicle (that I can think of right now).

@FreeMan why comment instead of answer?

@MarkJonesJr. because I only addressed one small part of the question in (as Delta Lima pointed out) a fairly superficial way. Also, I wouldn't even begin to be able to address the math as Peter Kämpf did in his answer.

@ymb1 yes, because when going in reverse, the weight shifts forward relative to the motion of the vehicle, putting more weight on the wheel(s) that are now in the front. So there, smart alek!  :D

@DeltaLima: Somehow I doubt that many aircraft have their wheels above their center of gravity.

Yes of course. And even with nose wheel brakes the main braking force would still be generated by the main wheels, so a substantial pitching moment is generated.
Now for a crude first-order approximation. We have an answer about the braking forces here, and using those 235 kN per wheel (the 747 has 16 main wheels) means that the center of gravity, which I guess to be at least 6 m above the ground, experiences inertial loads of 3,76 MN. This needs to be reacted in a moment around the main wheels with the lever arm of the wheel base. This is conveniently displayed in the drawing below and is 25.6 m. Thus, the reaction moment around the main wheels requires a nose wheel load of 881 kN in addition to the regular load of the nose wheel (which is maybe 5% of the aircraft's gross weight or 196 kN).

Boeing 747 side view (picture source)
Since the highest friction coefficient is reached at low speed, wing lift or tail downforce will not play a significant role. Braking to a full stop after an aborted take-off at full gross weight will load the nose wheels with a multiple of the static load of more than four.
In a normal landing, the forces involved are much lower and the multiple is close to one. This is really a quick back-of-the-envelope calculation, but the magnitude should still be right.

I would also point out that with regard to the extra load on the nose wheel, for a given deceleration it makes no difference how much of the braking force is provided by the nose wheel: since all wheels are the same distance below the line along which the aircraft's center of gravity travels, every newton of braking force on any wheel produces the same moment around the COG, and as this answer points out, that moment is what determines the load on the nose wheel.


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What is the fan tip speed at maximum thrust?
What is the fan tip speed at maximum thrust? Is it any different for a Rolls-Royce Trent 900 (powers the Airbus A380) and Pratt & Whitney PW6000 (powers the Airbus A318)?
Are they efficient at maximum thrust?


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What material was/is used on the skin of the Blackbird SR71?
What material was/is used on the skin of the Blackbird SR71 and how was it cooled.  How was it manufactured to be stronger with age

Titanium. Only the hottest portions of the fuselage (the chines) were cooled, using the fuel as a coolant. As far as I'm aware, there was no treatment to make the material stronger with age.

@securitydude5.  There's a great story that the Ti pieces made in summer failed structurally, but those in winter were ok. Eventually it was found chlorine was being added to the domestic water supply in summer to stop algae growth. Once they switched to distilled water, they had no problems. See page 11: https://www.cia.gov/library/readingroom/docs/CIA-RDP90B00170R000100050008-1.pdf

They found the heating and cooling caused by flying the aircraft improved the material strength with age.

Titanium is the only suitable material which can withstand those temperatures.
Stainless steel is much heavier. If you want to find out more about this read Ben Rich's book on the Skunk Works.

Did you mean to say titanium is not the only material?

Sorry it can be misunderstood. Stainless steel is also resistant to those temperatures however it is too heavy for flying at 70000 feet

Remember the u2 was built out of aluminium an extremely light material which made it very fragile. The incident with Powers wss caused when a missile's shockwave destroyed the aluminium, proof of how weak it was.

So, contrary to your claim, titanium is not the only material that can withstand those temperatures. It might be the only suitable material, but it's not the only material.


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Do pilots of airliners and jumbos do a walk around preflight check for every flight?
Do pilots of airliners and jumbos do a walk around preflight check for every flight?  How do pilots of large airliners and jumbos do sumping.  If its not a requirement because of aircraft design, why not implement the same design attributes to smaller and ALL other aircraft?

Sumping? You mean taking fuel samples?

Sampling to check for water accumulation in the fuel

Note that "being really big" is a design attribute, and implementation for small aircraft is not practical.

Modern jet transport aircraft have a water scavenge system which mixes water from the bottom of the tanks back into the fuel. Water draining is usually only necessary weekly. This system would incur too much weight and use too much energy on small aircraft versus the time it saves.

NO, not all airline pilots do a walk-around for every flight.
At my airline, maintenance personnel do a "Pre-Departure Check" (PDC) before each flight from all main bases on all wide body aircraft. (A330, B767, B777, and B787)
The PDC includes:
1) a walk around
2) a check of critical systems
3) a signature in the Journey logbook
As a result pilots very rarely do a walk around on the wide body aircraft.
If we happen to land somewhere without company maintenance, then a walk around would be done by the pilots. If it were an ETOPS flight, we would also have to make an entry in the Journey Log Book stating that the APU oil level was checked by noting the EICAS APU Oil Quantity reading.
In addition to the maintainance walk around, the ramp crew also does a quick walk around inspection just prior to push back.

Interesting, would you say that your airline is the exception rather than the rule? I know pilots from British Airways and Qantas who have to do walkarounds themselves even at their home base.

I honestly don’t know what other airlines do. I usually do long haul ETOPS flights and assume that other airlines would also require a maintainance sign off before an ETOPS flight. I would expect that maintainance persnonel would do a walk around during this inspection. Do British Airways and Qantas pilots also do walk arounds on ETOPS wide body flights?

Further on Ben's comment, I know Cathay Pacific pilots do that as well. I don't know the exact sign off procedure, but they have many long hauls and presumably ETOPS flight.

Certainly widebodies, ETOPS I don’t know for sure. I’ll add that it’s usually the cruise relief pilot who gets the job (especially when it’s raining...)

Yes, pilots of airliners do a walk around to visually check the aircraft before every flight, just like pilots of smaller planes do. A quick YouTube search brings up quite a few videos which explains the process.
The short version is, for items that are on the ground or within reach of the pilot, comparatively detailed checks are performed. For items that are beyond reach, a visual inspection is performed.
Due to the design of the aircraft, some checks that are possible on smaller aircrafts are impractical on airliners. For example you cannot move the flaps and elevators by hand. However, you can move them by using the yoke in the cockpit, once hydraulic power is on. There is a sensor which displays the position of the control surfaces in the cockpit. In a small aircraft, the pilot will move the yoke, then turn his head around to visually inspect the control surfaces are deflected to the correct direction.

"items that are on the ground or within reach of the pilot" For some planes not much falls into that category.

Every flight? Even if you have a very short turn-around?

A walk around is performed, but not necessarily by a pilot. See Mike Sowsun's answer.


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Are there any airliners and or aircraft that use bleed air and excess heat from the engine to prevent icing
are there any airliners and or aircraft that use bleed air and excess heat from the engine to prevent icing.  Rather than use battery power or alternators, this would make sense from an energy efficiency point of view because the use of alternators and batteries would use more fuels if providing electricity for heaters and recharging the batteries

The C-130 uses bleed-air for anti-ice. See Does the AC-130 have de-ice or anti-ice protection for wings and surfaces?

Quite a few actually, and this has been dealt with here before. Most systems use the bleed air from the engine, which is directed through pipes to wing leading edges, engine cowlings etc.
For example, the Boeing 767 anti-icing system, from this question:

Image from Aircraft Anti-Icing Systems by Leslie Mehl and Annie Parsons
This question has answers which explain about the anti-icing systems in large aircraft.

Boeing 727 anti-icing system, image from this answer